Generally, electronic equipment generates electrical noise, and equipment for which a countermeasure is insufficient is a factor in causing various problems. In particular, electrical equipment that operates on an alternating current power supply is often connected via a shared power supply line, and may experience a malfunction due to noise generated in other electrical equipment, or conversely, cause another device to malfunction due to noise generated in a device of the equipment itself.
As a source of noise, there is a slight electrical discharge in an electrical switch and various kinds of motor caused by a semiconductor such as an inverter, and the like, and weakening of energy generated in the source of noise, and causing transmission in a noise, transmission path to be difficult, are carried out as noise countermeasures.
Noise propagation is divided into two kinds depending on a transmission method thereof, wherein the first is normal mode noise generated between signal lines and between power supply lines, and is a noise component that is generated between power supply paths and flows in the same direction as a power supply current or signal. As noise current directions in the normal mode are of opposite orientations when going and returning, noise components are cancelled out and decrease, because of which there is little radiation noise, and this noise is countered by using filters in signal lines and power supply lines. The second is common mode noise generated between a signal line or power supply line and a ground, and this noise flows in the same direction to a signal pattern and SG (signal ground), passes through a metal frame, and returns to a signal source through a floating capacitance or the like.
Noise in the common mode is such that a large power supply loop is formed, which is important as a noise countermeasure as a large noise is radiated with even a small noise current. In response to this noise, filters are used between positive and negative signal lines and positive and negative power supply lines.
Patent Document 1 proposes a power circuit device that converts alternating current input from a system power supply into direct current, and subsequently into alternating current again, wherein, as a countermeasure for common mode noise, particularly in order to restrict radiation noise, a conductive plate is provided so that a common mode noise current is not caused to flow directly into a metal frame, the conductive plate and a bypass capacitor of a circuit configuring a power supply circuit are connected, and by the common mode noise current being taken in and connected to one point on the metal frame via an inductor, radiation noise is prevented from exiting directly from the metal frame.
Also, Patent Document 2 proposes using impedance elements to connect first and second radiators provided respectively in a first converter that converts alternating current into direct current and a second converter that converts direct current into alternating current, and reducing a wave height value of a common mode noise current by arranging that the common mode noise current always flows through the impedance elements.